This invention relates to apparatus that comprises support or carrier means to which is attached a mechamism having at least one element rotating at a fundamental or first-order frequency and one or more cyclically displaceable further elements directly or indirectly mechanically connected to said at least one element, the displacement of said cyclically displaceable element or elements producing a linear cyclic force of fundamental frequency and harmonics thereof acting on the same line and in which mechanism are rotary balancing means integral with or attached to said rotating element or elements. The invention is concerned with a method and means by which the transmission of the vibrations produced by such cyclic exciting forces to the support means can be reduced.
For convenience in the following description, said linear cyclic force of fundamental frequency together with any harmonics thereof acting along the same line will be referred to hereinafter as the "ACLEF" (being an abbreviation of "aggregate cyclic linear exciting force").
Many forms of apparatus are known that generate out-of-balance forces of this nature. In a very common example, the mechanism comprises a rotary crank, connecting rod and slider--the slider being a reciprocating piston--as in internal combustion engines, and fluid motors and compressors. In its simplest form with a single crank, connecting rod and piston, there is no difficulty in balancing the rotary forces generated by the movement of the crank, but to balance completely the reciprocatory linear cyclic force of the slider or piston involves either a multiplicity of sliders (e.g. six pistons in an in-line engine), or an integral mirror image construction (e.g. the Ariel "Square four" motor cycle engine), or a non-power contributing balancer piston, or one or more contra-rotating masses, or other additional moving elements. The co-linearity of the harmonics of out-of-balance reciprocating forces with the fundamental or first-order out-of-balance reciprocating force in crank/connecting rod/slider mechanisms is fully analysed in "A Treatise on Engine Balance using Exponentials" by P. Cormac, Chapter III, pages 14 and 15 (Chapman & Hall Ltd., London, 1923).
The line of action of the ACLEF may pass through the centre of gravity of the mechanism of which the cyclically displaceable elements are a part but more usually said line of action will be spaced from the centre of gravity of the complete mechanism that these elements are a part of, and there will be other elements, such as induction and exhaust means, a drive output transmission and the like, which cannot normally be disposed symmetrically.
It is generally accepted that the optimum result is obtained in practice by a compromise between balancing the rotary forces and balancing the linear cyclic forces. (E.g., see "Automobile Engineer's Reference Book" edited by Molloy and Lanchester, George Newnes Ltd., London 1956--Section 3 "Engines" pages 162-3 where, with reference to single-cylinder engines, it is stated that it is usual to balance the whole of the rotating weight and one-half of the reciprocating weight as this gives the best compromise: similarly, "Diesel Engines Principles and Practice" --George Newnes Ltd., London 1955, Section 13, pages 15-16 gives directions to a similar compromise in respect of single cylinder engines).
Having employed such a compromise for dealing in part with the fundamental order of the ACLEF by over-balancing the rotating masses, the result is an out-of-balance force of magnitude not less than half that of the fundamental order of the ACLEF and current practice is then to mount the mechanism on resilient means. A great variety of such resilient means have been devised and many are mentioned in "Suspension of Internal Combustion Engines" by M. Horovitz (Proc.I.Mech.E.(London) Automobile Division, No. 1, 1957-58, pages 17-51). There is no reference to the fundamental (or primary) out of balance except that in four-cylinder in-line engines it is generally small enough to be disregarded and the teaching is to minimise out-of-balance forces.
With regard to the use of resilient mounting means for piston engines, there are also various forms of mounting known for the purpose of isolating chassis suspension vibrations from the engine, as in the example of UK Pat. No. 1,200,191. These rely on mounting the engine so that it can pivot about an axis that is transverse to the crankshaft axis and that is some distance from the output end of the crankshaft. The arrangement is concerned with the problem of low frequency vibrations generated, for example, by movement of the vehicle over uneven ground and while it may effectively isolate such vibrations from the engine, it cannot be expected to have a substantial effect in preventing the transmission of engine vibration to the chassis.
There are many other instances in which the operation of a mechanism gives rise to an ACLEF. As further examples, applicances such as chain-saws and hedge-cutters can generate substantial ACLEFs in their operation.